Fabrication method for a 3-dimensional nor memory array

ABSTRACT

A process for manufacturing a 3-dimensional memory structure includes: (a) providing one or more active layers over a planar surface of a semiconductor substrate, each active layer comprising (i) first and second semiconductor layers of a first conductivity; (ii) a dielectric layer separating the first and second semiconductor layer; and (ii) one or more sacrificial layers, at least one of sacrificial layers being adjacent the first semiconductor layer; (b) etching the active layers to create a plurality of active stacks and a first set of trenches each separating and exposing sidewalls of adjacent active stacks; (c) filling the first set of trenches by a silicon oxide; (d) patterning and etching the silicon oxide to create silicon oxide columns each abutting adjacent active stacks and to expose portions of one or more sidewalls of the active stacks; (e) removing the sacrificial layers from exposed portions of the sidewalls by isotropic etching through the exposed portions of the sidewalls of the active stacks to create corresponding cavities in the active layers; (f) filling the cavities in the active stacks by a metallic or conductor material; (g) recessing the dielectric layer from the exposed sidewalls of the active stacks; and (h) filling recesses in the dielectric layer by a third semiconductor layer of a second conductivity opposite the first conductivity.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication (“Parent Application”), Ser. No. 16/914,089, entitled“Fabrication Method for a 3-Dimensional NOR Memory Array,” filed on Jun.26, 2020, which is a continuation application of U.S. patent applicationSer. No. 16/510,610, entitled “Fabrication Method for a 3-DimensionalNOR Memory Array,” filed on Jul. 12, 2019, which is related to andclaims priority of U.S. provisional patent application 62/697,085(“Provisional Application”), serial no., entitled “Fabrication Methodfor a 3-Dimensional NOR Memory Array,” filed on Jul. 12, 2018.

This application is also related to (i) copending U.S. patentapplication (“Copending Application I”), Ser. No. 15/248,420, entitled“Capacitive-Coupled Non-Volatile Thin-film Transistor Strings inThree-Dimensional Arrays,” filed Aug. 26, 2016 and published as U.S.Patent Application Publication 2017/0092371A1; and (ii) U.S. patentapplication (“Copending Application II”), Ser. No. 16/012,731, entitled“3-Dimensional NOR Memory Array Architecture and Methods for FabricationThereof,” filed on Jun. 19, 2018 and published as U.S. PatentApplication Publication 2018/0366489A1. The disclosures of theProvisional Application, the Parent Application, and the CopendingApplications I and II are hereby incorporated by reference in theirentireties. References to the Copending Applications I and II herein aremade by paragraph numbers of their respective publications.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to non-volatile NOR-type memory strings.In particular, the present invention relates to fabrication processesfor 3-dimensional arrays of non-volatile NOR-type memory strings.

2. Discussion of the Related Art

High-density structures representing arrays of generally non-volatilememory cells have been described in Copending Applications I and II. Thememory arrays of Copending Applications I and II are organized as stacksof connected storage transistors (“active stacks”) fabricated over asemiconductor substrate. Specifically, Copending Applications I and IIdisclose multiple strips of semiconductor layers (“active strips”) ineach active stack, with each strip providing storage transistorsorganized as NOR-type memory strings or “NOR memory strings”. Thesemiconductor substrate on which the memory array is constructed includevarious types of support circuitry, such as power supply circuits,address decoders, sense amplifiers, input and output circuits,comparators, and control and other logic circuits.

FIG. 1a illustrates schematically memory structure 100 containing NORmemory strings that can be fabricated using methods of the presentinvention. In this context, a NOR memory string consists of individuallyand independently addressable storage transistors sharing a commonsource region and a common drain region. As described in CopendingApplications I and II, each memory string may be formed along one sideof an active strip, which includes multiple layers of semiconductor andconductor materials. Memory structure 100 is organized as m activestacks each containing n active strips, where m and n can be anyinteger. For example, m may be 1, 2, 4, 8, 16, 32, 64, . . . , 8192 orgreater. Similarly, n may be 1, 2, 4, 8, . . . , 64 or greater.

As shown in FIG. 1a , memory structure 100 is represented by activestacks 130-(p−1), 130-p, 130-(p+1). In each active stack, n activestrips, labeled 101-1, 102-1, . . . , 101-n, are separated andelectrically isolated from each other by isolation layers 106. Isolationlayer 106 may be, for example, a silicon nitride. Each active stack iscovered on the outside by a layer charge storage material 121, which maybe provided, for example, by an oxide-nitride-oxide (“ONO”)triple-layer, as is known to those of ordinary skill in the art.Numerous conductive columns (not shown), separated from the activestrips by charge storage material 121, are provided in the space betweenactive stacks. These conductive columns provide gate electrodes, whichare used to select and operate the storage transistors formed along theactive strips on either side of the adjacent active stacks during read,write and erase operations. In the detailed description below, tosimplify the detailed description and for reference convenience, thedirection substantially perpendicular to the surface of thesemiconductor substrate (“vertical”) is labeled z. the direction alongthe length of each active strip is labeled y, and the direction alongthe width of each active strip is labeled x. The x and y directions arealso referred to as “horizontal”. One or more interconnect layers(“global interconnect layers”) may be formed above or below memorystructure 100 to provide conductors to interconnect the terminals of thestorage transistors in NOR memory strings of memory structure 100 tocircuitry in the semiconductor substrate.

Typically, one or more portions 108 in each active stack are dedicatedfor forming “staircase” or “reverse staircase” structures, which allowone or more of the semiconductor or conductor material layers in eachactive strip (e.g., the semiconductor layers providing a common drainregion or “bit line” in the active strip) to be accessed electricallyfrom the global interconnect layers, through conductors in vias (andburied contacts). In FIG. 1a , portions 108 (“staircase portions”) areprovided in the front and at the back of each active stack. Storagetransistors are formed in the portion or portions of the active strips(“array portion” or “array portions”) in each active stack outside ofthe staircase portion or portions. In FIG. 1a , array portion 109 isprovided between staircase portions 108.

FIG. 1b illustrates schematically the semiconductor and conductor layersof active strip 101. As shown in FIG. 1b , active strip 101 includes (i)n⁺ semiconductor layers 103 and 104 (e.g., n-type polysilicon) which mayprovide a common source region (or “source line”) and a common drainregion (or “bit line”) for a NOR memory string; and (ii) intrinsic orlightly doped p-type (p⁻) semiconductor layer 102, which may providechannel regions for the storage transistors of the NOR memory string.Between the dashed lines and separated from each active strip by chargestorage material layer 121 would be provided conductors (not shown) thatserve as gate electrodes for the storage transistors of the NOR string.The dashed lines in FIG. 1b indicate the positions of conductors122-(k−1), 122-k, and 122-(k+1) which are representative of suchconductors. In addition, as shown in FIG. 1b , conductor layers 105(e.g., tungsten with adhesion and barrier films) are provided adjacentn⁺ semiconductor layers 103 and 104. Conductor layers 105 reduceresistance in the common source and drain regions of the NOR memorystring. Isolation layers 106 (e.g., silicon nitride) electricallyisolate each active strip in the active stack from another.

The present invention provides a desired efficient process forfabricating memory structure 100.

SUMMARY

According to one embodiment of the present invention, a process formanufacturing a 3-dimensional memory structure includes: (a) providingone or more active layers over a planar surface of a semiconductorsubstrate, each active layer comprising (i) first and secondsemiconductor layers of a first conductivity; (ii) a dielectric layerseparating the first and second semiconductor layer; and (ii) one ormore sacrificial layers, at least one of sacrificial layers beingadjacent the first semiconductor layer; (b) etching the active layers tocreate a plurality of active stacks and a first set of trenches eachseparating and exposing sidewalls of adjacent active stacks; (c) fillingthe first set of trenches by a silicon oxide; (d) patterning and etchingthe silicon oxide to create silicon oxide columns each abutting adjacentactive stacks and to expose portions of one or more sidewalls of theactive stacks; (e) removing the sacrificial layers from exposed portionsof the sidewalls by isotropic etching through the exposed portions ofthe sidewalls of the active stacks to create corresponding cavities inthe active layers; (f) filling the cavities in the active stacks by ametallic or conductor material; (g) recessing the dielectric layer fromthe exposed sidewalls of the active stacks; and (h) filling recesses inthe dielectric layer by a third semiconductor layer of a secondconductivity opposite the first conductivity. In addition, an isolationlayer is provided to separate adjacent active layers.

According to one embodiment, the process further includes recessing themetallic or conductor layer from the exposed sidewalls of the activestep, wherein filling recesses in the dielectric layer also fillsrecesses in the metallic or conductor layer.

According to one embodiment, the process further includes: (a) removingthe silicon oxide columns prior to recessing the dielectric layer and(b) re-creating the silicon oxide columns after filling the recesses inthe dielectric layer by the second semiconductor layer.

In one embodiment, the process further includes providing a chargematerial over the exposed sidewalls of the active stack and forming wordlines by filling spaces surrounded by adjacent silicon oxide columns andadjacent active stacks with a conductor material. The thirdsemiconductor layer includes an in situ boron-doped polysilicon. In thatembodiment, (i) the first and second semiconductor layers of each activelayer respectively form a common drain region and a common drain regionof a plurality of storage transistors organized as a NOR memory string;(ii) the third semiconductor layer forms channel regions of the storagetransistors in the NOR memory string; and (iii) the word lines form gateelectrodes of the storage transistors in the NOR memory string.

According to one embodiment of the present invention, a staircasestructure for accessing one or more semiconductor layers in a3-dimensional memory structure includes: (i) providing a first activelayer; (ii) providing a first isolation layer on top of the first activelayer; (iii) providing a second active layer on top of the firstisolation layer, wherein the first and second active layers eachcomprise (a) a first semiconductor layer of a first conductivity; (b) adielectric layer of an insulative material underneath the firstsemiconductor layer; and (c) a second semiconductor layer underneath thedielectric layer; (iv) providing a second isolation layer on top of thesecond active layer; (iv) providing and patterning a photoresist layerover the second isolation layer to create an opening in the photoresistlayer, thereby exposing a first area of the second isolation layer; (v)anisotropically removing the exposed first area of the second isolationlayer and the portion of the second active layer under the first area ofthe second isolation layer so as to expose a first area of the firstisolation layer; (vi) recessing the photoresist layer to increase theopening in the photoresist layer, such that a second area of the secondisolation layer is exposed; (vii) anisotropically removing (a) theexposed first area of the first isolation layer and the exposed secondarea of the second isolation area, and (ii) the portions of the firstsemiconductor layer underneath the exposed first area of the firstisolation layer and the exposed second area of the second isolationarea; (viii) filling cavities created by the anisotropically removingsteps of (v) and (vii), using the insulative material; (ix) repeatingsteps (i) through (viii) a predetermined number of times; and (x)anisotropically removing the insulative material at predeterminedlocations to create via openings to reach the first semiconductor layerof two or more active layers.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates schematically memory structure 100 containing NORmemory strings of the type that can be fabricated using methods of thepresent invention.

FIG. 1b illustrates schematically the semiconductor and conductor layersof an active strip 101 in memory structure 100 of FIG. 1 a.

FIGS. 2(i), 2(ii), 2(iii), 2(iv), 2(v) and 2(vi) illustrate staircaseportions 108 of memory structure 100, in accordance with one embodimentof the present invention.

FIG. 3(i) is an x-z plane cross sectional view of array portion 109 ofmemory structure 100, showing patterned photoresist layer 205 defining a45-nm width for each active stack and 65-nm width for each trenchbetween adjacent active stacks.

FIG. 3(ii) shows resulting memory structure 100, with active stacks 207a-207 e, after the trenches are filled using the silicon oxide,photoresist 206 is removed, and the resulting surface planarized by CMP.

FIGS. 3(iii) and 3(iv) are top and x-z plane cross-sectional views,respectively, showing resulting memory structure 100 after etchingtrenches 209 for the word lines to be formed.

FIG. 3(v) shows resulting memory structure 100 after removal of SAC4layers 105 s-b and 105 s-t from the active stacks, thereby creatingcavities 211 in their place.

FIGS. 3(vi) and 3(vii) are top and x-z plane cross-sectional views,respectively, showing resulting memory structure 100 after trenches 209and cavities 211 are filled using a metallic/conductor material.

FIGS. 3(viii) and 3(ix) are top and x-z plane cross-sectional views,respectively, showing resulting memory structure 100 after bothmetallic/conductor layers 105 and spline oxide 102 o are recessed.

FIG. 3(x) shows an x-z plane cross sectional view through the word linetrenches, showing channel polysilicon 102 filling the recesses inmetallic/conductor layer 105 and spline oxide 102 o.

FIG. 3(xi) is a x-z plane cross sectional view of memory structure 100after charge storage material 213 and word lines 214 are deposited andplanarized

FIG. 3(xii) is a top view illustrating array portion 108 of memorystructure 100, after formation of a global interconnect layer.

FIG. 4 shows a top view of memory structure 100 after the word linespacer columns are patterned and etched.

In this detailed description, like elements in the figures are providedlike reference numerals to facilitate reference to features in thefigures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides efficient processes for fabricating amemory structure containing an array of NOR memory strings. In thedetailed description below, the parameters of each step (e.g.,temperatures, pressures, precursors, compositions and dimensions) areprovided for exemplary purposes only. Upon consideration of thisdetailed description, one of ordinary skill in the art will be able tomodify or vary these parameters without departing from the scope of thepresent invention.

According to one embodiment of the present invention, a process isprovided by which a memory structure containing NOR memory strings maybe formed over a planar surface of a semiconductor substrate. Initially,various types of support circuitry may be formed in—or at the surfaceof—the semiconductor substrate (e.g., power supply circuits, addressdecoders, sense amplifiers, input and output circuits, comparators, andcontrol and other logic circuits are fabricated).

An isolation layer (e.g., silicon oxide) may be formed on the planarsurface. Buried contacts may be formed in the isolation layer forconnection to the circuitry underneath. One or more global interconnectlayers may then be formed above the isolation layer. (In the followingdetailed description, referring to FIG. 2(i), these layers arecollectively referred to as substrate 150.)

Thereafter, base oxide film 107 (e.g., 50-nm silicon oxide film) isprovided. The semiconductor and conductor layers of an active strip(collectively, an “active layer”) are then provided. Multiple activelayers may be provided, layer by layer, with each active layer beingisolated from the next active layer by isolation films 106 (e.g., a30-nm nitride layer). In one embodiment, in order of deposition, eachactive layer may include (a) sacrificial layer 105 s-b (“SAC4 layer 105s-b”; e.g., a 40-nm layer of silicon germanium); (b) n⁺ dopedpolysilicon layer 104 (“drain polysilicon 104”; e.g., 30-nm in situarsenic-doped polysilicon film); (c) silicon oxide layer 102 o (“splineoxide 102 o”; 80-nm silicon oxide film); (d) n⁺ polysilicon layer 103(“source polysilicon 103”; e.g., 30-nm in situ arsenic-doped polysiliconfilm); and (e) sacrificial layer 105 s-t (“SAC4 layer 105 s-t”; e.g., a40-nm layer of silicon germanium). SAC4 layers 105 s-b and 105 s-t aresacrificial layers that would each subsequently be replaced by ametallic conductor layer, as discussed below.

During depositions of the active layers, staircase structures forelectrically accessing drain polysilicon 104 of each active strip to beformed are formed in staircase portions 108. Array portion 109 isprotected from the staircase formation steps by a mask over arrayportion 109. According to the present invention, the staircasestructures may be formed using one photolithography step for every twoactive layers. FIGS. 2(i) to 2(vi) illustrate staircase structureformation in staircase portions 108 of memory structure 100, inaccordance with one embodiment of the present invention.

FIG. 2(i) shows memory structure 100 after depositions of active layers101-1 and 101-2. Thereafter, photoresist layer 201 is deposited andpatterned over memory structure 100. A first etching step removes fromthe area not protected by photoresist 201, film by film, (a) isolationlayer 106 and (b) active layer 101-2 (i.e., “SAC4 layer 105 s-tb; sourcepolysilicon 103, spline oxide 102 o, drain polysilicon 104, and SAC4layer 105 s-b of active layer 101-2). This first etching step stops atisolation 106 immediately above active layer 101-1. The resultingstructure is shown in FIG. 2(ii).

Photoresist layer 201 is then recessed to further expose addition areasof active layer 101-2. The resulting structure is shown in FIG. 2(iii).Thereafter, a second etching step removes from the exposed portions ofisolation films 106 immediately above active layers 101-1 and 101-2, (a)SAC4 layers 105 s-t of both active layers 101-1 and 101-2, and (b)source polysilicon 103 of both active layers 101-1 and 101-2. Thissecond etching step stops at spline oxide layers 102 o of both activelayers 101-1 and 101-2. The resulting structure, which is a two-stepstaircase structure, is shown in FIG. 2(iv). Photoresist layer 201 isthen removed.

Silicon oxide 202 is then provided to fill the cavities created by thefirst and second etching steps. A following planarization step (e.g.,chemical-mechanical polishing (CMP)) planarizes the resulting surface.The resulting structure is shown in FIG. 2(v). Vias can then be createdin silicon oxide 202 and the underlying spline oxide 102 o of each ofactive layers 101-1 and 101-2 to allow access to drain polysilicon 104of each of active layers 101-1 and 101-2. These vias are created in asubsequent oxide etch after all active layers are deposited, asdiscussed below.

The steps discussed in conjunction with FIGS. 2(i) to 2(v) are repeatedevery two active layers deposited. Note that, these staircase formationsteps discussed in conjunction with FIGS. 2(i) to 2(v) require onephotolithography step every two active layers deposited, which is moreadvantageous than staircase formation steps used previously, whichrequire a photolithography step for every active layer deposited.

After all active layers of memory structure 100 are deposited and thecavities from the last first and second etching steps on the final twoactive layers are filled, an oxide etch may be performed at anappropriate time to create vias to reach drain polysilicon layer 104 ofeach active layer. The resulting structure is shown in FIG. 2(vi). InFIG. 2(vi) and in each figure discussed below, only for exemplarypurposes, four active layers 101-1, 101-2, 101-3 and 101-4 are shown. InFIG. 2(vi), vias 203-1, 203-2, 203-3 and 203-4 are vias illustrative ofthe vias that can be created to access the semiconductor and conductormaterial layers of the active layers present. One of ordinary skill inthe art would understand that the present invention is applicable to anystructure with any number of active layers and vias desired. When filledwith a conductor material (e.g., tungsten or p⁺ polysilicon), these viasprovide electrical connectivity between drain polysilicon 104 andcircuitry in semiconductor substrate 150 through the conductors in oneor more global interconnect layers to be formed above memory structure100.

After all the active layers are deposited, hard mask layer 205 isprovided over the active layers in array portion 109. Photoresist layer206 is the provided and patterned to define the active stacks andtrenches therebetween. FIG. 3(i) is an x-z plane cross-sectional view ofarray portion 109 of memory structure 100, showing patterned photoresistlayer 206 defining a 45-nm width for each active stack and a 65-nm widthfor each trench between adjacent active stacks. An etch through hardmask layer 205 and the active layers not protected by photoresist layer206 creates the active stacks and trenches therebetween. Silicon oxide208 is then deposited to fill the trenches. FIG. 3(ii) shows resultingmemory structure 100, with active stacks 207 a to 207 e, after fillingthe trenches using silicon oxide, removal of photoresist 206 andplanarization by CMP. Unless specified, all x-z plane cross-sectionalviews in FIGS. 3(i)-3(xii) are made in array portion 109 of memorystructure 100.

Silicon oxide 208 is then patterned and etched to define trenches to besubsequently filled by conductors (“word line trenches”). The remainingsilicon oxide 208 (“silicon oxide columns”) provide electricalinsulation between adjacent word line conductors. FIGS. 3(iii) and 3(iv)are top and x-z plane cross-sectional views, respectively, showingresulting memory structure 100 after etching word line trenches 209. InFIGS. 3(iii) and 3(iv) the word line trenches and remaining oxide columnas both 65 nm wide. The cross-section of FIG. 3(iv) is taken alongdashed line A-A′ shown in FIG. 3(iii).

Sacrificial SAC4 layers 105 s-t and 105 s-b in each active layer, whichare adjacent source polysilicon 103 and drain polysilicon 104,respectively, are next removed by a selective isotropic etchingtechnique. The isotropic etching proceeds laterally from word linetrenches 209 until all the sacrificial materials in SAC4 layers 105 s-tand 105 s-b are removed. During this process, the silicon oxide columnsprovide mechanical support to the active stacks (e.g., active stacks 207a-207 e of FIGS. 3(iv)). FIG. 3(v) shows resulting memory structure 100after all SAC4 layers 105 s-b and 105 s-t are removed from the activestacks, thereby creating cavities 211 in their place.

Cavities 211 and word line trenches 209 are then filled using ametallic/conductor material. The metallic/conductor material may beprovided by, for example, successive depositions of a barrier material(e.g., tungsten nitride or titanium nitride) and tungsten. FIGS. 3(vi)and 3(vii) are top and x-z plane cross-sectional views, respectively,showing resulting memory structure 100 after word line trenches 209 andcavities 211 are filled using a metallic/conductor material. In FIG.3(vii), cavities 211 in the active strips are replaced bymetallic/conductor layers 305. The x-z plane cross-sectional view ofFIG. 3(vii) is taken along dashed line B-B′ shown in FIG. 3(vi).

Thereafter, an anisotropic etch removes the metallic/conductor material305 from the word line trenches. In one embodiment, silicon oxidecolumns 208 continues to provide mechanical support after replacement ofthe SAC4 layers by metallic/conductor layers 305. Alternatively, thesilicon oxide columns can be removed by an anisotropic oxide at thistime.

A selective isotropic etches to recess metallic/conductor layers 105 andspline oxide 102 o can then be made. In one embodiment, the selectiveisotropic etch recesses each metallic/conductor layer 305 from thesidewalls of the active stacks by 5-6 nm. The isotropic oxide etchrecesses spline oxide 102 o by, for example, 5-6 nm from the sidewallsof the active stacks. If silicon oxide columns 208 are not removed, theisotropic oxide etch also recess the exposed side walls of silicon oxidecolumns 208 along they direction by the same amount on each side. FIGS.3(viii) and 3(ix) are top and x-z plane cross-sectional views,respectively, showing resulting memory structure 100 after bothmetallic/conductor layers 305 and spline oxide 102 o are recessed. FIG.3(viii) refers to the embodiment in which silicon oxide columns 208 areretained.

A lightly-doped p⁻ polysilicon (“channel polysilicon 102”) may then bedeposited to fill both the recesses in metallic/conductor layer 305 andspline oxide 102 o, and the word line trenches. An anisotropic etch ofchannel polysilicon 102 may be followed to remove channel polysilicon102 from word line trenches 209. Channel polysilicon 102 may be providedby deposition of in situ boron-doped polysilicon with a dopantconcentration, for example, at 5.0×10¹⁸ cm⁻³. FIG. 3(x) is a x-z planecross-sectional view through word line trenches 209, showing channelpolysilicon 102 filling the recesses in metallic/conductor layer 305 andspline oxide 102 o. One disadvantage from not removing silicon oxidecolumns 208 prior to the isotropic oxide etch is that the portions ofspline oxide 102 o behind silicon oxide column 208 are protected and notrecessed. As a result, channel polysilicon 102 deposited on an activestrip is not continuous along they direction, and each memory cell onthe active strip has its very small separate, discrete region of channelpolysilicon, which may not be efficient in providing sufficient chargecarriers during storage transistor operations (e.g., an eraseoperation).

If silicon oxide columns 208 are removed after the replacement of SAC4layers 105 s-t and 105 s-t by metallic/conductor layers 305, asdiscussed above, word line spacer columns may be provided at this timeby filling the trenches (resulting from removing silicon oxide columns208) between adjacent active stacks using a silicon oxide, patterningand etching the silicon oxide. The resulting word line spacer columnsare similar and are referred to, going forward, silicon oxide columns208, had they been retained. FIG. 4 shows a top view of memory structure100 after the word line spacer columns are patterned and formed. Tosimplify the following detailed description, these word line spacercolumns may be treated the same as silicon oxide columns 208 in theremainder of the processing. Accordingly, these word line spacer columnsare also labeled 208 and no further distinction is made between thesetwo sets of columns in the remainder of this detailed description.

Charge storage material 213 may then be deposited, which linesconformally the side walls of word line trenches 209. Charge storagematerial 213 may be achieved by successive depositions (in order) of: a2-nm thick tunnel oxide, a 6-nm thick silicon-rich nitride, a 6-nm thicksilicon oxide and 2-nm thick aluminum oxide (Al₂O₃). Thereafter, chargestorage material-lined word line trenches 209 can then be filled byconductor material 214 (e.g., p⁺ polysilicon) to provide the word lines.Planarization by CMP can then remove the excess charge storage material213 and conductor material 214 from the top surface of memory structure100. FIG. 3(xi) is a x-z plane cross-sectional view of memory structure100 after charge storage material 213 and word lines 214 have beendeposited and planarized.

Buffer oxide 215 may be deposited over memory structure 100, patternedand etched to provide contact vias to word lines 214 under buffer oxide215. These contact vias may be filled by tungsten (“tungsten plugs”).Tungsten plugs may also be provided at the same time in the vias made instaircase portions 108 to contact drain polysilicon or bit lines 104.Global interconnect lines may be provided above memory structure 100subsequently to interconnect word lines 214 and bit lines 104 tocircuitry in semiconductor substrate 150. FIG. 3(xii) shows a top viewof a portion of array portion 108 of memory structure 100. As shown inFIG. 3(xii), connections by global interconnect lines 216 to word lines214 are staggered (i.e., each of global interconnect lines 216 connectsevery other one of word lines 214; another global interconnect linesconnects the remaining word lines along the first global interconnectline).

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modifications within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

We claim:
 1. A process for forming a memory structure over a planarsurface of a semiconductor substrate, comprising: forming above thesemiconductor substrate a plurality of active stacks placedsubstantially at predetermined positions along a first direction that issubstantially parallel to the planar surface, separated one from anotherby a first electrically insulative material, each active stack extendinglengthwise along a second direction that is (i) substantially parallelto the planar surface and (ii) substantially orthogonal the firstdirection, wherein (i) each active stack comprises a plurality of activestrips, provided along at predetermined positions along a thirddirection that is substantially orthogonal to the planar surface, eachactive strip being separated one from another by a first isolationlayer; and (ii) each active strip comprises (a) first and secondsemiconductor layers of a first conductivity type, and (b) a secondisolation layer separating the first and the second semiconductorlayers; patterning and etching the first electrically insulativematerial to provide a plurality of openings at predetermined positionsalong the first direction, the openings each extending along the thirddirection, exposing sidewalls of the active stacks on opposite sides ofthe opening; etching the second isolation layer of each active stripthrough the openings, thereby creating recesses between the first andthe second semiconductor layers at opposite sides of the secondisolation layer; conformally depositing a third semiconductor layer, thethird semiconductor layer being of a second conductivity opposite thefirst conductivity, such that the third semiconductor layer lines thesidewalls of the opening and the walls of the recesses of each activestrip; anisotropically etching the third semiconductor layer to removethe third semiconductor layer from the sidewalls of the openings;conformally depositing a charge-trapping material on the sidewalls ofthe opening, such that the third semiconductor layer in the recesses arein contact with the charge-trapping layer; and filling the openings witha conductive material.
 2. The process of claim 1, wherein each activestrip further comprises a sacrificial layer provided adjacent each ofthe first and the second semiconductor layers on the side of thecorresponding semiconductor layer obverse to the second isolation layer,the process further comprising replacing the sacrificial layer by aconductive layer.
 3. The process of claim 2, wherein the conductivelayer comprises a barrier layer and a metallic layer comprising arefractory metal.
 4. The process of claim 3, wherein the barrier layercomprises tungsten nitride or titanium nitride.
 5. The process of claim4, wherein the refractory metal comprises tungsten.
 6. The process ofclaim 2, wherein replacing the sacrificial layer comprises isotropicallyetching the sacrificial layer from each of the openings using an etchantthat is selective to the first electrically insulative material.
 7. Theprocess of claim 2, wherein replacing the sacrificial material iscarried out prior to isotropically etching the second isolation layer ofeach active strip and wherein the process further comprising removingsubstantially all the first electrically insulative material prior toetching the second isolation layer.
 8. The process of claim 7, whereinthe second isolation layer comprises a second electrically insulativematerial, and wherein removing substantially all the first electricallyinsulative material comprises replacing the first electricallyinsulative material by a significantly less quantity of the secondelectrically insulative material, such that the openings aresubstantially enlarged.
 9. The process of claim 1, wherein the secondisolation layer comprises the first electrically insulative material,such that etching the second isolation layer also removes a portion ofthe first insulative material from the sidewall a of the opening. 10.The process of claim 1, wherein the first electrically insulativematerial comprises silicon oxide.
 11. The process of claim 1, whereinthe first isolation layer comprises silicon oxycarbon (SiOC).
 12. Theprocess of claim 1 wherein the second isolation layer comprises siliconoxide.
 13. The process of claim 1, wherein the third semiconductor layercomprises an in situ-doped polysilicon.
 14. The process of claim 1,wherein the third semiconductor layer has a dopant concentration that islower than the dopant concentration of either the first semiconductorlayer or the dopant concentration of the second semiconductor layer. 15.The process of claim 1, wherein conformally depositing thecharge-trapping layer comprises depositing a tunnel dielectric layer asilicon-rich nitride, and a blocking dielectric layer.
 16. The processof claim 15, wherein the blocking dielectric layer comprises an aluminumoxide layer.
 17. The process of claim 1, wherein the conductive materialcomprises heavily-doped polysilicon.
 18. The process of claim 1, furthercomprising forming conductors above the memory structure, each conductorextending along the second direction and in electrical contact withconductive material at predetermined locations.
 19. The process of claim1, wherein each active strip forms one or more NOR memory strings. 20.The process of claim 19 wherein, within each NOR memory stringscomprises a plurality of storage transistors, and wherein the first andthe second semiconductor layers provide a common drain region and acommon source region, respectively.
 21. The process of claim 20, whereinthe third semiconductor layer provides for each storage transistor achannel region.